M. Godinho

Preparation and neutron diffraction of superconducting “tetragonal” and non-superconducting orthorhombic Tl2Ba2Cu1O6

Abstract Many different samples of Tl 2 Ba 2 Cu 1 O 6 have been prepared by different heat treatments at relatively low temperature. The material as first prepared is clearly orthorhombic and non-superconducting, but on annealing and quenching, the samples become almost tetragonal with high T c . Various T c values between 3+ O 2- atoms are missing, creating the electron holes apparently necessary for superconductivity. The main difference between the various samples appears to be that the non-superconducting material is orthorhombic, with a well ordered superstructure, while the superconducting material is pseudo-tetragonal, with disordered oxygen within the TlO plane, as for the higher superconducting members of the Tl 2 Ca n Ba 2 Cu n +1 O 2( n +3 ) series. Superconductivity then appears to depend on the precise structural arrangement, and not just on stoichiometry and the number of electron holes.

Magnetic behaviour of γ-Fe2O3 nanoparticles by mössbauer spectroscopy and magnetic measurements

Abstractγ-Fe2O3 nanoparticles with varying state of dispersion in a polymer have been investigated by Mössbauer spectroscopy, static magnetic measurements at low applied field, and alternative susceptibility measurements over a large range of frequencies (2×10−2–104 Hz). The dynamical behaviour was characterized through the variation of the blocking temperature with the characteristic time of the measurement. The Mössbauer blocking temperature was determined according to a procedure described. For quasi-isolated particles an Arrhenius law is demonstrated. Effects of interparticle interactions in concentrated and aggregated systems are satisfactorily explained by the previous model. Dependence of the superparamagnetic susceptibility on the experimental conditions interpreted using the Lorentz or Onsager fields is mentioned.

Electron microscopy of superconducting “tetragonal” and non-superconducting orthorhombic Tl2Ba2Cu1O6

Abstract HREM of samples of orthorhombic, non-superconducting Tl 2 Ba 2 Cu 1 O 6 and “tetragonal” superconducting Tl 2 Ba 2 Cu 1 O 6 show s that the difference in orthorhombicity is associated with the degree of order of the superstructure. The superstructure modulation is unidirectional and is generated by the vector q =±0.07, 0.22, 1. This is not a principal direction of the sublattice. Twinning of the basic structure and superstructure results in small domains with eight different orientations. Since neutron diffraction shows that one in eight Tl and O atoms are missing in the TlO bilayers, a model is proposed for the superstructure with one in eight Tl and O vacancies aligned along [±130] directions as required by the HREM images.

Magnetism in Ar-implanted ZnO

ZnO single crystals were implanted with Ar ions with an energy of 100 keV and different fluences. Ferromagnetic behaviour is observed at room temperature after implantation. This behaviour is suppressed after consecutive annealings at 400 and 500 °C. Although trace transition metal impurities have been identified in the virgin samples, it is shown that they cannot account for the observed magnetic behaviour that is assigned to the presence of implantation-induced lattice defects.

Hyperthermia studies of ferrite nanoparticles synthesized in the presence of cotton

MFe2O4 (M = Co, Fe, Mn) compounds were synthesized using hydrothermal treatment in the presence of medicinal cotton. Two sets of nanoparticles were produced for each composition and subsequently characterized by XRD, TEM and SEM. The nanoparticles obtained from the solution display the expected spinel structure and different mean sizes (below 16 nm); the nanoparticles embedded in cotton were subjected to a calcination process for cotton elimination. Regarding these calcinated samples, the spinel structure was maintained for CoFe2O4, a mixture of phases was identified for the M = Mn sample and, in the case of iron, the magnetite phase was converted to hematite (α-Fe2O3). After cotton elimination the samples exhibit a morphology which evidences the role of cotton as a template. To evaluate the quality of the nanoparticles for hyperthermia, SQUID magnetometry and Mossbauer spectroscopy were used to perform the magnetic characterization of all products, and the specific loss power (SLP) was determined by induction heating measurements. All the ferrite NP obtained by hydrothermal synthesis in the presence of cotton display good hyperthermia performance. MnFe2O4 nanoparticles exhibit the highest SLP value, 90 W g−1, followed by Fe3−xO4, and CoFe2O4. In the case of CoFe2O4, the specific loss power of the NP obtained after cotton elimination is enhanced by 50% which is explained by the NP morphology adopted from the cotton template during the synthesis.

Influence of oxygen stoichiometry on the electronic properties of La4Ni3O10±δ

Some different oxygen content La4Ni3O10+δ (δ=0.02,0.12,−0.22) compounds were prepared in order to study the influence of Ni3+ content on the electronic properties. A metal–metal transition was detected at 140 K for the as-prepared (La4Ni3O10.02) and reduced (La4Ni3O9.78) compounds. From the electrical conductivity, Seebeck coefficient and magnetic studies of the different samples, oxygen stoichiometry was found to be a critical parameter in defining the existence of a less conductive low temperature state. The results are interpreted assuming the existence of charge order in the nickel oxide planes of the layered perovskite-type structure.

UFe6Ge6: a new ternary magnetic compound

Abstract The ternary intermetallic magnetic compound UFe6Ge6 was studied by single crystal X-ray diffraction and 57Fe Mossbauer spectroscopy; the magnetic properties were also investigated. UFe6Ge6 was found to melt congruently at c. 950 °C and to crystallize in space group P6/mmm, with cell parameters a = 5.126 8(4) A , c = 4.050 7(5) A , V = 92.204 7(13) A 3 , Z = 0.5 and a YCo6Ge6 type structure that was solved by single crystal X-ray diffraction to a final R = 0.059 (Rw = 0.071). Magnetization measurements suggest two transitions: one at 322(5) K reminiscent of an antiferromagnetic ordering and another ferromagnetic at 230(10) K. According to 57Fe Mossbauer spectroscopy there is a transition at 322(5) K associated with the ordering of the Fe moments which do not undergo any noticeable reordering at lower temperatures. Thus the transition at 230 K is tentatively attributed to the ordering of the U-sublattice.

Electronic structure, optical and magnetic properties of Co2FeGe Heusler alloy films

Optical properties of ferromagnetic half-metallic full-Heusler Co

57Fe Mössbauer spectroscopy study of the AFexAl12−x intermetallics (AY, Tm, Lu and U, 4≤x≤4.3)

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Magnetic properties of Fe100−xSmx thin films and Fe80−xSmxB20 thin films and ribbons

FeGe alloy are investigated experimentally and theoretically. Co